Recently, the number of the people suffering from circulatory diseases such as myocardial infarction and cerebral infarction has been increasing. How to prevent and treat such diseases is an important issue. To the onset of myocardial infarction and cerebral infarction, arteriosclerosis is deeply involved. Specifically, where atheroma is formed on a blood vessel wall or new cells of an artery stop being generated due to any of various factors such as high blood pressure and the like, the artery loses resilience thereof and becomes hard and brittle. As a result, the blood vessel is occluded at the site where the atheroma is formed, the blood vessel tissue covering the atheroma is ruptured and the atheroma flows into the blood vessel and occludes the artery at another site, or the hardened site of the artery is ruptured. These cause the above-described diseases. Therefore, it is important to provide an early diagnosis of arteriosclerosis in order to prevent or treat these diseases. A method or apparatus for providing a diagnosis of the progress level of arteriosclerosis on an early stage is desired.
Conventionally, a diagnosis of an arteriosclerosis lesion is provided by directly observing the inside of a blood vessel using a vascular catheter. However, this method of diagnosis requires the vascular catheter to be inserted into the blood vessel and has a problem of imposing a heavy bodily burden on a test subject. Therefore, the observation with a vascular catheter is used on a test subject who certainly has an arteriosclerosis lesion for the purpose of specifying the site thereof, but is never used for a medical checkup.
Measuring a level of cholesterol, which is a cause of arteriosclerosis, or measuring a blood pressure level does not impose a heavy burden on a test subject and is easy to conduct. However, these levels do not directly indicate the progress level of arteriosclerosis.
Providing a diagnosis of arteriosclerosis on an early stage and administering a therapeutic drug of arteriosclerosis to the test subject is effective to treat the arteriosclerosis. However, once the arteriosclerosis progresses, it is considered to be difficult to completely cure the hardened artery by a therapeutic drug although a further progress could be suppressed by the drug.
For these reasons, a method or apparatus for providing the progress level of arteriosclerosis on an early stage with little burden on the test subject is desired.
As a noninvasive medical diagnostic apparatus which does not impose a heavy burden on a test subject, an ultrasonic diagnostic apparatus or an x-ray diagnostic apparatus is conventionally used. By irradiating the body of a test subject with an ultrasonic wave or an x-ray from outside the body, information on the shape inside the body or information on the time-wise change of the shape can be provided without causing a pain to the test subject. Once the information on the time-wise change of the shape of the measurement target inside the body (motion information) is obtained, the information on the properties of the measurement target can obtained. For example, the elasticity of the blood vessel is found based on a tiny change of the thickness of the blood vessel, which is superimposed on a motion with a large amplitude resulting from the heartbeat, namely, a distortion amount of the blood vessel, and also based on the blood pressure difference. Accordingly, by obtaining the motion information, the elasticity characteristic of the blood vessel in a biological body is found and so the level of arteriosclerosis can be directly found.
Especially, ultrasonic diagnosis realizes the measurement merely by applying an ultrasonic probe to the test subject, and so is superior to the x-ray diagnosis in that administration of a contrast medium to the test subject is not needed and there is no risk of exposure to the x-ray radiation. A conventional ultrasonic diagnostic apparatus provides a tomogram showing the structure of a test subject by converting the intensity of an echo signal into the luminance of the corresponding pixel. The tomogram is provided on a real-time basis and is used to diagnose the structure of the inside of the test subject.
The recent development of electronic technologies is rapidly improving the measurement precision of ultrasonic diagnostic apparatuses. In accordance with this, ultrasonic diagnostic apparatuses for measuring the tiny motions of tissues of the biological body are now under progressive development. Measurement of the tiny motions of the tissues of the biological body at a high precision can provide a detailed two-dimensional distribution of the elasticity characteristic of the arterial wall.
For example, Patent Document No. 1 discloses a technology of tracking the measurement target at a high precision by analyzing the amplitude and phase of an ultrasonic echo signal using the constrained least squares method. This technology is called the “phase-difference tracking method”. This technology can measure, at a high precision, a vibration component which is caused by the blood vessel motion and has an amplitude of several microns and a frequency of up to as high as several hundred hertz. It is reported that this technology makes it possible to measure the thickness change or distortion of the blood vessel wall at a high precision on the order of several microns.
Patent Document No. 2 discloses a technology for scanning a plurality of scanning zones defined for a test subject with an ultrasonic wave and measuring the elasticity characteristic of the blood vessel in each scanning zone.
Patent Document No. 3 discloses an ultrasonic diagnostic apparatus which measures a characteristic of the blood vessel which is different from the elasticity characteristic, specifically a value representing the thickness of the carotid artery, as an index used for determining whether or not the test subject has arteriosclerosis. The carotid artery is known to include three layers of an intima, a media and an adventitia from the inside. The ultrasonic diagnostic apparatus described in Patent Document No. 3 measures the total thickness of the intima and the media (intima-media thickness; hereinafter, referred to as “IMT”).
The ultrasonic diagnostic apparatus described in Patent Document No. 3 does not include means for measuring the displacement (distortion) of the blood vessel and so cannot measure the elasticity characteristic thereof. This ultrasonic diagnostic apparatus absolutely needs to have a function of providing a three-dimensional display of the blood vessel before the IMT value is measured. The processing of providing such a display is time-consuming and cannot avoid increasing the cost.
Patent Documents Nos. 4 and 5 each disclose a technology for finding a value representing the shape of the blood vessel wall using the technology of Patent Document No. 1 and calculating the elasticity characteristic. Patent Document No. 6 discloses a technology for providing a three-dimensional image of the shape of the blood vessel and finding the thickness of the blood vessel wall at an arbitrary cross-section thereof from the obtained three-dimensional image.    Patent Document No. 1: Japanese Laid-Open Patent Publication No. 10-5226    Patent Document No. 2: Japanese Laid-Open Patent Publication No. 2001-292995    Patent Document No. 3: Japanese Laid-Open Patent Publication No. 2006-000456    Patent Document No. 4: International Publication No. 2006/011504 pamphlet    Patent Document No. 5: International Publication No. 2006/043528 pamphlet    Patent Document No. 6: Japanese Laid-Open Patent Publication No. 2006-456